Cerium oxide is an important oxide of rare earth elements, and it has many excellent properties as a catalyst support or an additive to supports, which have been confirmed in catalysis researches. In subsequent researches, it is reported that the cerium oxide can exhibit unique advantages as a catalyst support in oxidative reaction, methanol cleavage and reductive reaction of nitrogen oxides due to good performances in oxygen storage and release and strong redox behaviors between trivalent cerium ions and tetravalent cerium ions.
The physical and chemical properties of cerium oxide greatly depend on the microstructure thereof, e.g., size, morphology, and specific surface area, etc. Cerium oxide materials having micro- and nano-scale porous structure not only can well satisfy requirements of highly effective absorbing materials on the microstructure thereof, but also can increase catalytic active sites of surface by increasing specific surface area, thereby to increase catalytic properties of the cerium oxide materials. Hence, researches for the preparation of such kind of materials have great realistic significance. However, it is found that nano- and/or micro-scale cerium oxide particles and cerium oxide aerogel having a large specific surface area as prepared according to existing methods known in the art, as catalyst supports, have poor mechanical strength and resistance to high temperature, and are easy to be sintered and collapse in hollow structure at a high temperature in use.
When being applied in the field of catalytic after-treatment to exhaust emissions containing CO, NO or volatile organic compounds, supported catalysts as prepared by using cerium oxide having conventional structures as the catalyst support would involve the following defects: 1) Hydrothermal stability is poor. The support per se will easily take sintering and collapsing in hollow structure at a high temperature, which will decreases the surface area and porosity. 2) Active components, e.g., noble metals or transition metals, as supported by the support will be easily embedded during the hydrothermal aging at a high temperature. 3) Sulfur resistance is poor, and if the catalyst is exposed to a sulfur-containing atmosphere for a long period, the catalyst will be deactivated.
The above defects seriously restrict the applications of cerium oxide materials in the field of catalytic after-treatment of exhaust emissions that includes a stationary source and a mobile source. Hence, at now, it is in interest that how to maintain a high specific surface area of a nano-scale cerium oxide support while assuring its mechanical strength and resistance to high temperature.